A hard disk drive (HDD) is typically equipped with a magnetic recording medium and a magnetic head, and the magnetic head reads and writes data on the magnetic recording medium. The magnetic head in the HDD typically includes a write head for writing information on the magnetic recording medium (magnetic disk) as magnetic signals and a read head for reading out signals recorded on the magnetic recording medium as magnetic signals. The read head typically includes a magnetoresistive effect laminated body having a plurality of magnetic thin films and non-magnetic thin films and it is called a magnetoresistive effect head because it reads signals by utilizing the magnetoresistive effect.
There are several kinds of laminated structures for magnetoresistive effect heads and they are classified into categories such as an AN MR head, a GMR head, a CPP-GMR head, and a TMR head in accordance with the principle of the magnetic resistance used therein. They use magnetoresistive effect (AN MR), giant magnetoresistive effect (GMR), current perpendicular to plane GMR effect (CPP-GMR effect), tunnel magetoresistive effect (TMR), respectively, and transduce input magnetic fields entering the read head from the magnetic recording medium into voltage variations.
Nowadays, development in high recording density has created a requirement for a reading scheme with higher sensitivity. In a range of 500 (Gb/in.2) to 2 (Tb/in.2), the TMR which has a very high MR ratio is advantageous in view of an improvement of sensitivity. For ultra high recording density exceeding 2 (Tb/in.2), the CPP-GMR or the like is expected to be the main type used. Being different from the current in plane GMR (CIP-GMR) in which the sense current flows parallel to the film planes of the magnetoresistive effect stacked body, the TMR and the CPP-GMR are schemes in which the sense current flows perpendicular to the film planes, i.e., in the direction of stacking of the film planes. In the present specification, the scheme like this is referred to as a CPP scheme; and a read head using a CPP scheme is referred to as a CPP read head.
The magnetoresistive effect laminated body typically includes a fixed layer whose magnetization direction is fixed and a free layer whose magnetization direction changes with external magnetic fields. The magnetoresistance change and output increase with the spin polarizability of the free layer. One type of half-metals with a 100% or almost 100% spin polarizability is Heusler alloy. It is proposed to use a Heusler alloy in a free layer in Japanese Unexamined Patent Application Publication No. 2008-227457.
If a Heusler alloy is used in a free layer, however, the magnetostriction is so large that the element easily becomes unstable. Thus, Japanese Unexamined Patent Application Publication No. 2008-227457 proposes a free layer structure with a Heusler alloy layer, a soft magnetic layer and a magnetostriction reduction layer between them. The magnetostriction reduction layer consists of elements of the fourth group, the fifth group or the sixth group. It indicates that the free layer structure achieves a high MR ratio and low magnetostriction.
Japanese Unexamined Patent Application Publication No. 2008-227457 discloses a magnetostriction reduction layer made of a Hf film, a Ti film, a Zr film, a Ta film or a W film. If an intermediate layer is formed between a Heusler alloy layer and a soft magnetic layer, however, it is important to consider the magnetic coupling between the Heusler alloy layer and the soft magnetic layer.
If the intermediate layer between the Heusler alloy layer and the soft magnetic layer is made of a non-magnetic material like the material disclosed in Japanese Unexamined Patent Application Publication No. 2008-227457, the magnetic coupling of the Heusler alloy layer and the soft magnetic layer is severed, so they do not work integrally but work independently. Thus, the magnetic volume of each of the layers is reduced compared to the layers working integrally, resulting in the increase of magnetic fluctuations caused by thermal excitation. The fluctuations increase noise in read operations of the read head and error rates. Thus, it is preferable that the intermediate layer has a property such that the Heusler alloy layer and the soft magnetic layer work integrally. Namely, non-magnetic materials are not preferred for the intermediate layer and it is preferably made of a magnetic material.
Accordingly, a technique is desired to accomplish a high MR ratio and low magnetostriction in a CPP magnetic read head with a free layer including a Heusler alloy layer and provide a CPP read head having a superior noise property as well.